Communications transceivers such as telephones, fax machines and voice modems, that are designed to work on copper twisted pair wire communication lines and transmit signals in the 200 Hz to 4 kHz frequency range support the connection of more than one transceiver at a remote customer end of the line. This is generally possible through the definition of two states of such equipment called the On-hook and Off-hook states. These are derived from conventional telephony terms that indicate whether the handset of the telephone is on or off hook. Equipment in the Off-hook state is active and participates in a communication session. Equipment in the On-hook state is inactive and does not participate in a communication session, has negligible impact on the line, and can be ignored by other equipment connected to the line. Typically, at any given instant, only one piece of equipment connected to the line is in the Off-hook state, while others are in the On-hook state.
Although work on DSL transceivers dates back to the late nineteen-eighties and early nineteen-nineties, present Digital Subscriber Line (DSL) transceivers and other high speed communication devices are not designed to support the connection of more than one transceiver or transmitter at any given end of the line. Additional high speed communication devices, or DSL transceivers, which is one type of high speed communication device, cannot be connected to either end of the line without incurring a significant loss in the quality of transmission and reception of signals.
Examples of high speed communication systems, such as DSL communications systems, are Asymmetric DSL (ADSL) and Very high speed DSL (VDSL). xDSL equipment can generally be broken down into two basic units, the xDSL Transceiver Unit Central Office (xTU-C) and xDSL Transceiver Unit Remote (xTU-R).
In theory, a copper twisted pair of wires of infinite length has a characteristic impedance for a given frequency. In practice, however, the copper twisted pair of wires has a finite length and a normal impedance. The normal impedance typically has a slightly lower impedance than the characteristic impedance but is substantially equal to the characteristic impedance. In this disclosure, the terms normal impedance and characteristic impedance are used interchangeably.
DSL transceivers in use are only capable of presenting an impedance that is substantially equal to the characteristic impedance of the copper twisted pair of wires over the transceivers' frequency range of operation. This impedance is generally 100 Ohms. Ordinarily, this ensures that echo is kept at a minimum and maximum power transfer is effected into or from the line, as shown in FIG. 1. This result does not follow if more than one transceiver is connected at any end, in parallel.
Several problems arise if more than one DSL transceiver presently in use is connected at one end of the line. For example, problems related to attenuation, echo, contention, and non-deterministic impedance when powered-off may arise. This is because, typically, the additional transceiver or transceivers offer the same impedance, i.e. the normal impedance, across the line.
First, the additional transceiver(s) causes signal power attenuation. Specifically, the power of the signal received from the other end of the line is divided into each of the transceivers at the end where more than one transceiver is connected (parallely connected transceivers). At the parallely connected end, as shown in FIG. 2, each transceiver receives less power than it would have received if it were the only one connected at that end.
FIG. 6 summarizes simulations that have been performed on SPICE to measure the impedance presented to the line when two ATU-Rs are connected in parallel. As shown in FIG. 6, the power of a signal received by an ATU-R is reduced by about 4.8 dB if another ATU-R, which also has a termination of 100 Ohms, is connected in parallel. Here the loop has not been included. Depending on the loop length, the power reduction can be up to 6 dB for one additional ATU-R.
As shown in FIG. 7, the impedance presented to the line at the parallely connected end (the effective line impedance) is further reduced when more than one ATU-R, each terminated with the normal impedance of 100 Ohms, is connected in parallel. Because the line is now terminated with a lower impedance than the characteristic or normal line impedance, the signal power transfer from the transmitter at the other end of the line into the line will be less than the possible maximum (as per the Maximum Power Transfer Theorem: Schaum's Outline Series Theories And Problems Of Electric Circuits By Joseph A Edminister, Published By McGraw Hill Book Company (August 1965). This reduced power then travels along the line and further gets distributed amongst the ATU-Rs connected at the end of the line as mentioned above.
Further, as shown in FIG. 3, each transceiver connected in parallel to a transmitting transceiver acts as a load on signals transmitted by the transmitting transceiver. Hence, the total power transmitted onto the line is less than the power that would have been transmitted if only one transceiver was connected at the transmitting end of the line. The transceiver at the other end of the line (distant-receiving transceiver), therefore, receives signals having significantly reduced power than the distant-receiving transceiver would have received if a single transceiver were connected at the transmitting end of the line.
Second, the connection of additional transceivers in parallel also results in increased echo. Presently, transceivers are designed to transmit onto a line terminated at the other end by the characteristic impedance. When more than one transceiver is connected to a transmitting end of the line in parallel, as depicted in FIG. 4, the impedance presented to the transmission line by all the transceivers connected in parallel at the transmitting end of the line is much less than the characteristic or normal impedance. This will result in echo at the transmitting end of the line.
Echo, at any interface, is dependent on line termination impedance, and generally the effectiveness of echo suppression and cancellation circuits is dependent on the termination presented by the interface. Connection of more than one transceiver on the line changes this termination and thus increases the echo. FIG. 8 summarizes the results of a simulation and shows the drastic increase in the echo seen by the transmitting ATU-R when additional ATU-Rs with a termination of 100 Ohms are connected in parallel.
As shown in FIG. 5, additional transceivers connected in parallel at a receiving end will also present impedance that is less than the characteristic line impedance to signals arriving at the receiving end of the line. This will cause a larger reflection of the signal back into the line resulting in a larger echo at the interface ‘R’, resulting in an increased echo at the distant-transmitting transceiver.
The transceivers will hence be subject to higher echo, causing complications associated with increased echo in the performance of the transceivers. Echo, if not cancelled or suppressed, would reduce the effective dynamic range of the receiver and could, in a worst case scenario, saturate the receiver altogether. Reduction of the dynamic range could adversely affect the performance of the receiver resulting in lower bit rates and saturation could cause non-linear distortion also resulting in lower bit rates.
Simulations have been performed on MATLAB, for a specific test case (Case#3 in Annex D of the ITU-T G.992.2 standard), which involves the loop T1.601#7 (13.5 kft plain loop) and a 24 DSL NEXT (Near End crosstalk) and a −140 dBm/Hz background noise (ITU-T Recommendation G.992.1 (06/99), “Asymmetric Digital Subscriber Line (ADSL) Transceivers”, International Telecommunication Union). One active ATU-R modem communicating with the ATU-C and a number of passive ATU-Rs connected at the remote end have been simulated. The SNR profile is simulated for one ATU-R and for each additional passive ATU-R (max of four). The results are shown in FIG. 12. From this figure we see that the SNR profile drops drastically, even if one additional ATU-R is added.
The following table shows the bit rates generated for the above SNR profiles, after giving 4 dB margins.
Number ofAdditional ATU-RsBit Rates (kbps)02208114402112039284800
Third, when more than one transceiver has to be connected on the same medium, contention problems arise. Contention is the inadvertent, simultaneous transmission of similar signals by more than one transmitter onto the same transmission medium, at the same end in particular. This could result in signal corruption.
It has been suggested, in line with the prevalent method of avoiding contention in data communication systems, that the transceivers could check or sense the medium to determine whether the medium is idle before actually transmitting signals. This is possible if the receiver can probe for signals on the line without loading existing signals, if any, on the line to avoid corrupting existing signals and to prevent disrupting communication in progress between other transceivers. However, if a transceiver that has a normal impedance probes the line, it will also load the line. Therefore, it is desirable for a receiver to present a high impedance across the line to probe. Existing DSL transceivers are not capable of presenting a high impedance across the line while receiving the signal and hence could disrupt on-going communication.
Fourth, the impedance of existing DSL transceivers, when powered off, is not known. This is because the network that constitutes the impedance seen by the line may contain active devices, and the characteristics of active devices cannot be determined in the power-off condition. Therefore, if a current DSL transceiver remains connected to the line when powered off, it could be presenting a low impedance across the line and hence, any of the other transceivers connected to the same line are not guaranteed to work at their rated performance level or may not work at all. Even if transceivers had a mechanism incorporated to ensure automatic line disconnection when powered off, they would immediately get connected to the line and offer normal impedance when switched on, and could thus potentially disturb ongoing communication on the line. The user would have to ensure that other modems connected to the line are not communicating before switching the modem on. Therefore the mechanism stated above, although automatic in one sense, would not actually serve its purpose.
Given the problems that arise when more than one DSL transceiver, presently in use, is connected across the same line, it has been suggested that the only way to resolve the aforementioned problems is to check and ensure that all other transceivers are disconnected from the line before using any one transceiver on the line. All users would have to carry out this cumbersome procedure whenever they want to use their DSL modem. As explained above, even powering off the other transceivers may not help. In practice it would render the use of multiple DSL transceivers on the same line very unwieldy as compared to the use of voice band transceivers, the predecessors of the DSL transceivers. It should be noted that although the above discussion refers to DSL transceivers, the problems noted above apply to other high speed communication devices as well, such as high speed communication transmitters and co-axial modems for example. Accordingly, there continues to be a need for a high speed communication device that resolves the problems mentioned above.